Serial bus arrangements such as CAN (Controller Area Network), FlexRay and the like are widespread in various technical fields, for example automotive engineering, for the purpose of networking control units. The CAN bus operates, for example, according to the so-called multi-master principle and connects a plurality of control units having equal rights. A CSMA/CR method resolves collisions, that is to say simultaneous bus access operations, without damaging the winning, higher-priority message. For this purpose, the bits are dominant or recessive (a dominant bit overwrites a recessive bit), depending on the state. In this case, the logic 1 is recessive (wired AND). The bus can be physically configured using copper lines or glass fibers, for instance.
Data buses of the above-mentioned type must be very stable on account of the interference immunity required in the automotive environment, in particular, and are therefore generally in the form of a linear bus. In this case, the bus is terminated at both physical ends of its geometrical extent with a respective terminating resistor (bus termination) matched to the characteristic impedance of the bus lines. As a result, reflections produced on the bus lines in the case of higher-frequency data transmission rates can be minimized and the bus can be operated in an optimum manner. Since the CAN bus system is a serially operating two-wire bus with an open architecture which predominantly uses TP (twisted pair) cables with a nominal impedance of 120 ohms as the transmission medium, a conventional terminating resistor or a conventional bus termination in the case of a CAN bus is approximately 120Ω at each of the two ends of the bus.
Since the baud rates of current bus systems are moving toward ever higher values on account of their real-time capability and increasing bus load, a correct bus termination is absolutely necessary for reliable communication.
On account of different levels of equipment of respective vehicles or as a result of the same control unit or the same ECU (Electronic Control Unit) being installed in different vehicle types, the need to respectively arrange or omit the above-mentioned terminating resistors may exist at different locations of the bus inside a vehicle network. In other words, a terminating resistor must be present on the basis of a location at which an ECU is connected to the bus, or said terminating resistor can or must be dispensed with.
Arrangements which can comprise terminating resistors which can be connected manually, for example, by wire bridges, bridges in a cable harness and the like are inflexible and disadvantageous in so far as changes to the characteristic data of a bus cannot be excluded and a function cannot be ensured without any influence on the bus communication.